Motorbike Routes? Thinking Hoi An - Hanoi?

My boyfriend and I will be in Vietnam for three weeks. We are meeting friends in Hanoi the last week so we have two weeks to explore. We want to rent/buy motorbikes and travel on our own for at least a week. Since we don't have enough to do the entire length from Ho Chi Minh to Hanoi I was think about flying from HCM up about half way and getting a bike there. Has any had any experience with this? Recommendations on where to start etc? Once we make it up to Hanoi we will sell the bikes and take a train to explore SaPa As well as ha long Bay. I would appreciate any advice on this or Vietnam in general!

I am a bit anxious about the road crowded with people in Vietnam so I am not interested in exploring from North to South by motorbike. Sa Pa is a good place to visit, especially if you come here in mid-September - early October.